[FIX] models: don't assume ids are ints in sorted()

Some models (e.g. calendar.event), use "virtual" ids which are not represented
as integers. It was not possible to use sorted method on those models as calling
int() is failing.

This commit fixes the method, making it agnostic to the type of the
'id' member variable.

Fixes #7454

openerp/models.py

@@ -50,6 +50,7 @@ import re
 import time
 from collections import defaultdict, MutableMapping
 from inspect import getmembers, currentframe
+from operator import itemgetter
 
 import babel.dates
 import dateutil.relativedelta
@@ -5391,7 +5392,7 @@ class BaseModel(object):
             recs = self.search([('id', 'in', self.ids)])
             return self.browse(reversed(recs._ids)) if reverse else recs
         else:
-            return self.browse(map(int, sorted(self, key=key, reverse=reverse)))
+            return self.browse(map(itemgetter('id'), sorted(self, key=key, reverse=reverse)))
 
     def update(self, values):
         """ Update record `self[0]` with ``values``. """